1. Field of the Invention
The present invention relates to an electrically rewritable nonvolatile memory, for example, a flash EEPROM or other semiconductor nonvolatile memory device.
2. Description of the Related Art
As one type of electrically rewritable nonvolatile memory, there is known for example a NOR type flash memory. This NOR type flash memory writes data by using the channel hot electron (CHE) effect to inject electrons into a floating gate and erases data by using Fowler-Nordheim (FN) tunneling to pull the electrons from the floating gate to the source.
However, a NOR type flash memory has the problem of "over-erasing" in an erase operation. For example, when erasing all of the memory cells at one time, the erasing pulses continue to be supplied until erasing all of the memory cells while repeating a read operation for verifying the erasure. However, the erasure characteristics of the memory cells differ due to manufacturing variations etc. If the erasing pulses continue to be supplied until the memory cells which are the slowest in erasing have finished being erased, the memory cells which are the fastest in erasing will be over-erased. In certain circumstances, there is a possibility that the threshold voltage Vth of those memory cells will be depleted.